The instant invention relates to stabilized power supply, so-called switching power supplies.
A switching power supply operates in the following way. A transformer primary winding receives a current which is for example supplied by a rectifying bridge receiving energy from the mains. The current in the transformer is chopped by a switch (for example a power transistor) in series with the primary winding.
A transistor control circuit establishes periodical square pulses for setting on the transistor. During the occurrence of the square pulses, the current is allowed to flow; outside those square pulses, the current flow is not allowed.
An ac voltage is then obtained on one (or several) secondary winding(s) of the transformer. This voltage is rectified and filtered for supplying a d.c. voltage which is the output d.c. voltage of the switching power supply.
In order to stabilize the value of this d.c. voltage, one acts upon the duty cycle of the switch periodical conduction, that is, upon the ratio between the ON and OFF state durations within a switching period.
FIG. 1 shows an examplary switching power supply architecture manufactured by the Applicant wherein two integrated circuits are used. One of those circuits, CI1, serves to control the base of a power transistor Tp for applying switching ON and switching OFF periodical signals thereon. This base control circuit CI1 is positioned on the primary winding side (EP) of the transformer (TA). The other integrated circuit, the regulation circuit CI2, is on the contrary positioned on the secondary side (winding ES1) and it serves to determine the output voltage VS1 of the power supply for elaborating regulation signals that it transmits to the first integrated circuit through a small insulating transformer TX. The first integrated circuit CI1 uses those regulation signals for modifying the duty cycle of the switching transistor Tp and therefore adjusting the output voltage Vs of the power supply.
Referring to FIG. 1, one can see the supply transformer TA, the primary winding EP of which is fed in A1 by an electric current, supplied for example by a rectifying bridge connected to the mains. The other extremity of the primary winding EP is connected to the collector of a switching transistor Tp, the emitter of which is connected to the primary ground (designated in the drawings by a black triangle).
The transformer is provided with several secondary windings which are preferably insulated from the mains and connected for example to a secondary ground insulated from the primary ground.
Each of the secondary windings has an extremity connected to the secondary ground. The other extremity of each secondary winding feeds a low-pass filtering capacitor through a rectifying diode.
Later on, only one secondary winding ES1, connected through a diode 10 to a capacitor 11, will be considered. The d.c. voltage of the switching power supply is the voltage VS1 through the terminals of capacitor 11 ; but, of course, other output d.c. voltages are liable to be obtained through the terminals of other filtering capacitors connected to secondary windings through rectifying diodes. Those output voltages constitute stabilized supply voltages for utilization circuits, not shown. By way of example, a secondary winding ES2 supplies a supply voltage stabilized at a few volts for feeding the regulation integrated circuit CI2 hereinabove mentioned. The circuit CI2 receives on its input the voltage VS1, the value of which is compared with a reference value applied to a pulse-width modulator which sets variable width periodical square pulses as a function of the output voltage value VS1; the lowest the VS1 value is, the larger the width of the square pulses will be.
The square pulses are set to the switching frequency of the switching power supply. This frequency is thus set on the secondary circuit side; it is generated either inside the circuit CI2, or at the outside of a circuit not shown, in the form of a saw-tooth voltage at the chosen switching frequency, connected to the circuit CI2 on a terminal 12. This saw-tooth voltage is besides used in a known manner for determining the pulse-width modulation (PWM).
The variable width square pulses supplied by the regulation circuit CI2 at its output 13 are applied to a primary winding 14 of a small transformer TX, the secondary winding 15 of which, insulated from the primary, supplies variable width square pulses. Those pulses, having a position and a frequency determined by the regulation circuit CI2, are the regulation signals applied to an input 16 of the control circuit CI1.
The transformer TX is constituted by a few windings rolled on a ferrite rod, the windings of the primary and those of the secondary being sufficiently spaced apart from one another to comply with the standards of galvanic insulation between the primary and secondary circuits of the switching power supply.
The control integrated circuit CI1 comprises means known per se designed to constitute at its output 17 variable width periodical square pulses precisely reproducing the signals supplied in 13 by the regulation circuit CI2. Those signals supplied in 17 by the circuit CI1 are applied to the base of the switching transistor Tp.
The circuit CI1 is energized in A3 by any known means, and is connected to the primary ground.
Such a switching power supply is liable to be advantageously used in a TV set. In such a case, it is advantageous that the saw-tooth voltage applied in 12 to the circuit CI2 be synchronized with the horizontal scanning frequency so that the switching transistor Tp be switched ON at the spot retrace, in order that the spurious pulses generated by the switching ON of the transistor do not appear on the picture.
Another mode of regulating a switching power supply is shown in FIG. 2. This other regulation mode is called primary regulation because it is operated through a circuit CI3 located on the primary side.
In FIG. 2, one can see, similarly to FIG. 1, the supply transformer TA comprising a primary winding EP fed by a rectified current and connected to the collector of a switching transistor Tp, the emitter of which is connected to the primary ground. Each of those secondary windings ES1 and ES2 supplies electric currents which, after rectification and filtering, constitute the power supplies one wishes to get stabilized. The regulation system is constituted by another secondary winding ES3, an extremity of which is connected to the primary ground and the other extremity to the input 18 of the circuit CI3. The voltage applied on the input 18 is compared, in the circuit CI3, with a reference value and is applied on a pulse-width modulator which sets variable width electrical square pulses as a function of the voltage value applied on 18. The square pulses are set to the switching frequency of the switching power supply. This frequency is therefore et on the primary side of the circuit; it is generated in the form of a saw-tooth voltage at a chosen switching frequency.
This primary regulation system presents the advantage of being simple to implement, since it does not necessitate either an auxiliary transformer TX or a shaping circuit for the transformer output signal.
But, such a primary regulation system presents the drawback of a poorer regulation quality, as well as major difficulties for obtaining a synchronization between the switching frequency and another frequency resulting from the operation of a device energized on the secondary side, for example of a device corresponding to the horizontal scanning of a TV set.
For various reasons, especially resulting from the choice to be made between the regulation performances and the cost of the components, it appears more and more advantageous, for example for a TV set, to use a switching current comprising both a primary and a secondary regulation system. As regards a TV set, the primary regulation is used when the set is operating in a standby mode, and the secondary regulation is used when the TV set is operating in a normal mode. The primary regulation is set so as to stabilize the power supply at a value lower than that of the secondary regulation. Thus, when the TV set is normally operating, one of the outputs VS1 of the stabilized power supply is liable for example to supply a 124 volts voltage, and when the TV set is in the standby mode, said output VS1 is liable to supply a voltage of about 100 volts. Of course, the other outputs VS2, etc., provide voltages that vary in the same ratios according to the operating mode.
If a switching power supply is thus comprising both a primary and a secondary regulation, a switching power supply system has to be provided comprising not only the circuits such as described in FIG. 1 but also those described in FIG. 2. It is noticeable, in such a case, that the base of the switching transistor Tp is to be connected both to an output 17 of the circuit CI1 and to an output 19 of the circuit CI3, for example through an OR gate.
When the TV set is in the standby mode, the primary regulation device only is operating and the circuit CI3 alone supplies the signals to the base of the switching transistor Tp. When the TV set is operating normally, the primary and secondary regulation devices are simultaneously operating, but since the primary regulation is set in order to stabilize a voltage lower than that of the secondary regulation, the circuit CI3 does not supply any output signal since, at its input 18, it detects a higher voltage than the reference voltage, and the secondary regulation circuit CI2 alone supplies output signals provided for controlling, through the circuit CI1, the base of the switching transistor Tp.
But, such a switching power supply system, which seems to operate properly in a stable standby or normal operation mode, presents faulty operations during the transition phase from one operating condition to another or if an overvoltage occurs during its functioning, causing the output voltage to rapidly drop. Indeed, during those transition phases or during those abrupt and accidental overvoltages, a simultaneous supply of signals from the primary and secondary regulation circuits is liable to occur for a short lapse of time on the base of the switching transistor Tp. The simultaneous occurrence of signals from the primary and secondary regulation circuits (which are not synchronized), is liable to cause the switching ON of the switching transistor Tp at periods when the demagnetization of the transformer TA is not completed; and it is known, in such a case, that the switching transistor is exposed to be damaged due to the fact that the switching ON is carried out while the collector is at a too high voltage and the switching transistor is, besides, liable to allow a higher current than the normal current to flow.
The instant invention relates to a device permitting, in any circumstances, on the one hand, to ensure the full demagnetization of the supply transformer TA between successive switching ON transitions and, on the other hand, to avoid too high currents to flow.